1. Field of the Invention
The present invention relates to a light leakage detecting device of an optical transmission system using a fiber optic cable, which device is used in an optical transmission system in which high-power light is outputted from a light guiding end of a fiber optic cable, and is used to detect light leakage in the optical transmission system.
2. Description of the Related Art
A technique has been developed in which a sheet-like recording material, particularly, a printing plate having a photosensitive layer formed on a support is used, and an image is recorded directly on the photosensitive layer (an emulsion surface) of the printing plate by means of a laser beam or the like (a printing plate exposure device). This kind of technique allows rapid recording of an image on a printing plate.
In an automatic printing plate exposure device using a technique for recording an image on a printing plate, an image is recorded on a printing plate in such a manner that, with the printing plate being wound on a peripheral surface of a rotating drum, a recording head (an exposure head) is moved along an axial direction of the rotating drum (sub-scanning) while the rotating drum is being rotated at a high speed (main scanning).
A photosensitive material such as the aforementioned printing plate has a density corresponding to an amount of exposure. Therefore, in order that the photosensitive material may have a density in a predetermined density range, one of increase or decrease in the quantity of light, or increase or decrease in exposure time is selected. In a scan-recording system in which a recording head is moved while a rotating drum is being rotated, the exposure time is limited because of high speed processing. Therefore, a high-power LD is used as a light source because it can emit a large quantity of light to accomplish the aforementioned purpose.
Further, in the automatic printing plate exposure device, a plurality of (for example, 50 or thereabouts) light sources are arranged in a sub-scanning direction so as to allow higher speed processing. As a result, sub-scanning by a plurality of light sources becomes possible by one revolution of the rotating drum.
However, in a case in which a plurality of light sources are used, all of the light sources cannot be mounted on a recording head which moves in a sub-scanning direction (because of increase in weight, or the like). Therefore, a light source unit is provided separately from the recording head and light from the light source unit is guided to the recording head via a connector and an optical fiber.
In this case, it is necessary to sufficiently give attention to leakage of light from a high-power LD. Therefore, an output state of the LD is monitored at predetermined intervals. That is, a light-quantity detecting sensor is disposed at an end surface of a rotating drum, and when a recording head is located at a home position, the LD is made to output light for inspection and the outputted light is compared with a predetermined threshold. As a result, it is determined as to whether leakage of light occurs or not.
However, in a conventional method, a high-power LD is made to output light in an approximately full-power state and the outputted value is compared with a threshold. Consequently, when an optical fiber may be broken or a connector may not be set (including imperfect mounting), leakage of light can be merely recognized only after high-power light has leaked. As a result, the leaked light has adverse effects on the inside and outside of the exposure device.
In order to solve the aforementioned drawback, it suffices that an output value for inspection is simply decreased (for example, 100 mW or less). However, in this case, output characteristics vary with the passage of time or due to an ambient temperature. As a result, a driving current required for obtaining a predetermined light level or variation of output light quantity with respect to current variation (an inclination of current vs. light output characteristics) changes. Further, after an LD is exchanged due to a failure, there is also a difference in output characteristics between high-power LDs before and after the replacement.
For example, when each level used to obtain an output value for inspection is set in accordance with an LD having an approximately upright inclination (high angle inclination) in output characteristics thereof, there is a possibility that no output value for inspection is obtained from an LD having an approximately horizontal inclination (low angle inclination) in output characteristics thereof.
On the other hand, when each output value for inspection is set in accordance with an LD having an approximately horizontal inclination in output characteristics thereof, an output for inspection from an LD having an approximately upright inclination becomes large. If leakage of light may occur in the case of the LD having an approximately upright inclination, this leakage of light is large enough for adversely affecting the device.